Update a running Proxmox Virtual Environment 3.x to latest 3.4

Make sure that you have uploaded a valid subscription key to your Proxmox VE host. All updates should be applied via CLI only (if you tried via GUI, please check this forum post). here is the howto for the CLI:

If you run windows KVM guests your operating system will recognize some hardware changes on the first boot and windows shows a new network card in the device manager. (e.g. you need to reassign the fixed IP setup to the new network card. if you have DHCP setup there should be no issues). Under some circumstances reactivation of your windows license can be necessary.